Photosensitively opacifiable glasses or glasses which can be thermally opacified after being exposed to ultraviolet radiation were initially disclosed over 35 years ago. Such glasses have been referred to by the shorthand term of photosensitive opal glasses. U.S. Pat. No. 651,145 (Stookey) describes such glasses consisting essentially, expressed in terms of weight percent on the oxide basis, of 55-75% SiO.sub.2, 12-18% R.sub.2 O, wherein R.sub.2 O consists of 0-2% Li.sub.2 O, 5-18% Na.sub.2 O, and 0-13% K.sub.2 O, 2-12% Al.sub.2 O.sub.3, 0.005-0.05% CeO.sub.2, 0.0001-0.3% Ag computed as AgCl, and, as analyzed, 1.8-3% fluorine, and the indicated proportion of a halogen selected from the group consisting of 0.01-2% chlorine, 0.02-0.4% bromine, and 0.03-0.6% iodine, the sum of those constituents composing at least 85% of the total composition. As extraneous components the patent noted that BeO, MgO, and CaO should not be present in amounts greater than 3%, either separately or collectively. Up to 12% BaO, SrO, and ZnO may be included either separately or collectively. Up to 5% CdO may be added. Nevertheless, the total of all of the above divalent metal oxides should not exceed 12%.
Materials strongly absorbing of ultraviolet radiations should be avoided. Explicit reference was made to glass colorants such as selenium and its compounds, and oxides of iron, copper, uranium, and vanadium, as well as the noncoloring oxides of arsenic, lead, and thallium.
The patent also noted the utility of up to 0.2% Sb.sub.2 O.sub.3 or up to 0.1% SnO.sub.2 for increasing the photosensitivity of the glass. Greater levels of each were observed to destroy photosensitivity, however. Because of the concomitant fining action exerted by Sb.sub.2 O.sub.3, its inclusion was preferred to that of SnO.sub.2.
The mechanism providing opacity to the glasses was defined in the following terms. The glasses as melted and shaped were clear and transparent, and will remain so when merely reheated. Exposure to short wave radiations, however, preferably those having wavelengths between 3000-3500 .ANG. (ultraviolet radiations), produces an invisible latent image therein. Thus, exposure to ultraviolet radiation causes the photolytic reduction of Ag.sup.+ ions to silver metal (Ag.degree.), as exemplified by the following reaction ##STR1## wherein hv represents a photon of electromagnetic radiation. That latent image, present only in the exposed portions of the glass, is converted to a visible opaque image upon a subsequent three-stage heat/cool treatment.
In the first stage the exposed glass was heated for a time and at a temperature varying from about one minute at 50.degree. C. above the softening point of the glass to about one hour at about 150.degree. C. below the softening point of the glass. Temperatures lower than 150.degree. C. below the softening point were cited as being ineffective, and temperatures higher than 50.degree. C. above the softening point were asserted to be both impractical and detrimental to the image. It was postulated that this initial heat treatment caused the development of submicroscopic nuclei of colloidal silver, but with no visible change in appearance. (It was observed that, where the content of silver exceeded about 0.002%, the silver nuclei generated were of a size and number sufficient to impart a yellow coloration to the glass.)
In the second stage the glass article was cooled to a temperature at least below 500.degree. C. No visible change occurs in the glass but it was conjectured that submicroscopic nuclei of the opacifying agent, i.e., an alkali metal fluoride, are formed on the colloidal silver nuclei as a result of the cooling. The degree of cooling below 500.degree. C. was not critical, e.g., the glass could be cooled to room temperature, but the step of cooling below 500.degree. C. was absolutely vital to achieve the desired opacification.
In the third stage the glass article was reheated to a temperature not lower than about 100.degree. C. below the softening point of the glass for a sufficient period of time to cause the fluoride nuclei to grow and form opacifying crystallites. The opacifying crystallites develop only in the irradiated portions of the glass article.
Photosensitively opacifiable glasses prepared in accordance with the method outlined in U.S. Pat. No. 2,651,145 containing NaF crystallites as the opacifying phase have been marketed commercially by Corning Incorporated, Corning, New York for more than 30 years under the trademark FOTALITE.RTM.. One such glass which has been marketed as flat panels for architectural applications as Corning Code 8607 consists of the following approximate composition, analyzed in terms of weight percent:
______________________________________ SiO.sub.2 70.4 F 2.5 Ag 0.0008 Na.sub.2 O 16.36 Sb.sub.2 O.sub.3 0.20 *Co.sub.3 O.sub.4 0.005 ZnO 5.0 Br 0.5 *NiO 0.05 Al.sub.2 O.sub.3 8.0 CeO.sub.2 0.015 SnO 0.01 ______________________________________ *Colorants to impart a light gray tint thereto.
In commercial manufacture, the heat treatment applied to the glass subsequent to exposure to ultraviolet radiation has utilized a three-stage schedule comprising heating to about 540.degree. C. and holding at that temperature for about one hour, cooling to a temperature below about 300.degree. C., reheating to about 580.degree. C. and holding thereat for about one hour, followed by cooling to room temperature.
In general, the glass panels fabricated for architectural applications have wall thicknesses about 1/4" (.apprxeq.6 mm). As can be appreciated, to assure that the panels exhibit essential total opacity there is the need for the opacifying crystallites to be present in substantial concentrations throughout the full thickness of the panel wall. An extremely practical problem that has been present in the above composition is the very long period of exposure to ultraviolet radiation which the panels require in order to achieve dense opacification throughout the cross section thereof. Hence, periods of exposure in excess of one hour to a Hg-Xe arc lamp at 1000 watts power have proven to be necessary.
Although it would seem that the photographic speed of a photosensitive opal glass is a simple number, inversely proportional to time, in point of fact several factors must be studiously controlled to optimize sensitivity. These factors include:
(1) the need for the photographic pattern to uniformly penetrate the full thickness dimension of the glass; i.e., to assure minimum attenuation of ultraviolet radiation through the glass;
(2) the integrated sensitivity as a function of ultraviolet radiation wavelength, thickness of the glass, and optical absorption spectrum of the glass;
(3) the optical emission spectrum and intensity of the exposing light source; together with the absorption spectrum of any filter or negative film interposed between the light source and the glass; and
(4) the temperature of the glass during exposure to ultraviolet radiation.
Nevertheless, it was appreciated that each of those factors could be influenced by glass composition. Therefore, the primary objective of the invention was to develop transparent glass compositions requiring much shorter periods of exposure to ultraviolet radiation of a given intensity in order to demonstrate essentially total opacification throughout the thickness dimension of a glass article.